Method of chemical nickeling and cadmium chemical plating of metallic and nonmetallic substrates

ABSTRACT

A METHOD OF APPLYING SMOOTH NICKEL OR CADIUM COATINGS TO METALLIC OR NONMETALLIC (E.G., FABRIC) SUBSTRATES IN WHICH THE DEGREASED AND DEOXIDIZED SUBSTRATE SURFACE IS TREATED WITH AN AQUEOUS SOLUTION OF A CADMIUM OR NICKEL SALT (E.G., THE METAL SULFATE OR CHLORIDE) AND SODIUM HYPOPHOSPHITE WHEREBY, AFTER THE ADDITION TO THE CHEMICAL-PLATING BATH OF AN ORGANIC ACID (PREFERABLY FORMIC ACID), THE BATH IS RAISED FROM ITS PH OF ABOUT 2 WITH A BASIC COMPOUND (E.G., NH4OH) TO APPROXIMATE NEUTRALITY (PH OF ABOUT 6.5 TO 9) AND IS STABILIZED THEREAFTER BY A COMPLEX FORMER OF THE ETHYLENEDIAMINETETRAACETIC ACID (EDTA FAMILY).

United States Patent O 3,565,667 METHOD OF CHEMICAL NICKELING AND CAD- MIUM CHEMICAL PLATIN G F METALLIC AND NONMETALLIC SUBSTRATES Carl Klingspor, 17 Waldstrasse, 59 Siegen, Westphalia, Germany No Drawing. Filed Nov. 8, 1967, Ser. No. 681,568 Int. Cl. B44d 1/09; C230 3/00 US. Cl. 117-47 7 Claims ABSTRACT OF THE DISCLOSURE ity (pH of about 6.5 to 9) and is stabilized thereafter by g a complex former of the ethylenediaminetetraacetic acid (EDTA family).

My present invention relates to a method of chemically plating metallic and nonmetallic substrates without an external electric current and, more particularly, to a method for the cadmium and nickel plating of synthetic resin, ceramic, metallic and other substrates with continuous and adherent coatings.

The chemical plating of a substrate without the application of an electrode current has already been proposed, especially in connection with the formation of nickelsurfacing layers on metallic substrates. While a number of plating systems has been provided heretofore, they are generally characterized by the provision of a reduceable nickel salt and a reducing agent which, in the solution, chemically reacts with the nickel salt to form elemental or metallic nickel which deposits upon the surface of the substrate immersed in the bath. Such systems are generally of the type using a highly alkaline solution or acidic solution of a nickel salt, usually nickel citrate, complex formers, and aliphatic saturated short-chain monocarboxylic acids with 3 to 5 carbon atoms, at a pH of 3.5 to 6. These systems have, however, certain characteristic disadvantages based in part upon the acidic or basic character of the treatment solution. Thus acidic baths of the general character described solubilize iron and other metals from the substrate and result in the inclusion of impurities in the nickel coating. Chloridecontaining baths for chemical-plating purposes may result in the formation of free chlorine or chlorine compounds which may endanger the quality of the nickel coating. Strong alkali baths have similar disadvantages.

I have found that it is possible to limit the solubilization of impurity metal, to limit the formation of chloridecontaining or other impurity in the metal coating, and to control simply and effectively, while obtaining excellent coverage and coating continuity, when the degreased and deoxidized surface of the metallic or nonmetallic substrate is treated with an aqueous solution of a nickel or cadmium salt and sodium hypophosphite to which has previously been added an organic acid to bring the solution to a pH value of about 2, this acidic solution being then neutralized by the addition of an alkaline substance and stabilized by the addition to the bath of a complex-forming agent of the ethylenediaminetetraacetic-acid class. Preferably, the nickel-salt solution is chloride-free.

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According to a more specific feature of this invention, the bath is an aqueous solution of nickel sulfate or cadmium chloride and sodium hyposulfite in approximately chemically equivalent quantities, while the complex former is the disodium salt of ethylenediaminetetraacetic acid and the alkali with which the solution is brought to approximate neutrality is ammonia or ammonium hydioxide. The organic acid is advantageously formic acid. The expression approximate neutrality as used herein is intended to indicate a pH ranging from about 6.5 to about 9, although best results are obtainable at about pH 8 to 9. The chemical-plating operation is carried out at a temperature of 40 to C. and preferably about 55 C. while solution is circulated through a filter to remove particulate materials.

When the method is employed with metallic or metalized substrates, conventional degreasing and deoxidizing techniques, e.g., mechanical abrasion, pickling and the like may be employed, concurrently with or followed by etching. When nonmetallic substrates are provided, it is preferred to use a metalizing pretreatment. In the case of ceramic and other porous refractory bodies (bodies which can be heated to a temperature of about 200 C. without damage), I prefer to treat the body with a solution of the nickel salt and sodium hypophosphite and an organic acid in an organic solvent and thereafter to heat the body dryness at approximately 200 C. to form finely divided metal within and upon the body. The pretreatment solution has an organic solvent, preferably methanol, while the organic acid most advantageously is formic acid. I have also found it to be desirable to pretreat nonmetallic substrates with zinc chloride and a silver salt solution to provide a surface that is highly receptive to the chemical plating of nickel or cadmium.

According to a further feature of this invention, the substrate may be a synthetic resin in the form of, for example, a cast or molded solid synthetic-resin body (e.g., of ABS or polyester resin), a synthetic resin film or coating upon some other body, a fabric composed of synthetic-resin (e.g., polyamide) fibers or filaments in woven or nonwoven form, a laminate or the fibers and filaments themselves.

PREPARATION OF CHEMICAL-PLATING BATH (A) A nickel-deposition bath is prepared by dissolving 2185 kg. nickel sulfate heptahydrate (NiSO -7H O), 3.1 kg. sodium hypophosphite (NAH PO 10 liters formic acid, 25 liters ammonium hydroxide (N'H OH) in the form of a 25% aqueous solution, and 1.2 kg. of Titriplex III, a complexing agent produced by E. Merck, Darmstadt, Germany, and constituting the disodium salt of ethylenediaminetetraacetic acid in suflicient water to produce 100 liters of the bath.

The bath is formed generally by dissolving the nickel sulfate in Water in an amount of 25 to 30 grams per liter and adding to this solution the sodium hypophosphite in approximately the equivalent quantity necessary to reduce the nickel sulfate, i.e., about 30 to 35 grams per liter sodium hypophosphite of the nickel sulfate solution. Prior to, during or subsequent to the addition of the sodium hypophosphite, formic acid is added to the solution (approximately 100 ml. per liter of an aqueous solution with a titer of formic acid) until the solution has a pH of about 2.

The neutralization is effected preferably with ammonium hydroxide, although sodium hydroxide may also be employed to a pH value of 5.5. At this point, ammonium hydroxide only is used to complete the neutralization to the aforedescribed pH value of 6.5 to 9 and preferably a pH between 8 and 9. Thereafter, the Titriplex III is added.

The bath produced in this manner is heated to a temperature of 40 to 70 C., preferably about 55 C., to produce a wear-resistant ductile nickel coating upon metallic substrates or nonmetallic substrates pretreated as described below.

(B) Cadmium chemical plating is carried out by substituting 2.8 kg. of cadmium chloride for the 2.85 kg. of nickel sulfate in the bath (A) while otherwise following the procedure outlined in the preparation of the nickel-plating bath (A). The cadmium-plating bath thus has the following composition (per 100 liters of the bath) 2.8 kg. cadmium chloride, 3.1 kg. sodium hypophosphite, 10 liters formic acid, 25 liters ammonium hydroxide (25% aqueous solution) and 1.2 kg. of Titriplex III (disodium salt of ethylenediaminetetraacetic acid).

PREPARATION OF SUBSTRATE (I) A nonmetallic-filament substrate consisting of Woven or nonwoven polyamide fabric whose yarn is monofilamentary or polyfilamentary and which is composed of Perlon or nylon is dipped into a chromic acid bath consisting essentially of 200 grams chromium trioxide and 14 kg. concentrated sulfuric acid for a period of 2 to minutes at a temperature between 60 and 80 C. (the time of treatment depends on thread gauge and treatment temperature). The web is then rinsed in water, dipped in hydrochloric acid for 0.5 to 1 minute and rinsed again in water. The web is then dipped into a zinc-chloride solutiton for 1.5 to 2 minutes, in a silverchloride solution for 2 minutes, rinsed again thoroughly and dried. The resulting web can be nickelor cadmiumplated as described below at a temperature of 55 C. and receives a bright coating in a period of 12 to 20 seconds using the nickel and cadmium chemical-plating baths (A) or (B) described above. Depending upon the thickness of the web and the fiber or filament diameters, the plating time can range from 1 to about minutes with corresponding increase in thickness of the coating.

The thread can also be coated by passing it continuously through the chromic acid bath followed by the subsequent metal chloride and silver solution treatment described and nickel or cadmium chemical plating in the manner described.

(II) Synthetic resin plates of phenol-formaldehydeimpregnated laminates (e.g., paper) are cadmiumor nickel-plated after a preliminary treatment as follows:

(a) The synthetic-resin laminate is roughened with sandpaper of a grain size of 220 and is then cleaned by immersing it for a period of 2 to 5 minutes in a concentrated sulfuric acid bath at a temperature of 45 to 60 C. The result is a fat-free surface.

(b) The degreased phenol-formaldehyde laminate is then rinsed for a period of 2 minutes in cold water containing the usual detergents.

(c) The workpiece is then dipped for 10 seconds in a hydrochloric acid solution (1:1 in water), followed by rinsing in water.

The workpiece is then immersed in a sensitizing bath containing a metal chloride (e.g., Zinc chloride) in 10 to 20% by weight solution for a period of 1 to 1.5 minutes at room temperature. The sensitizing is followed by a thorough rinsing in water. Prior to nickelor cadmiumchemical plating, the workpiece is immersed in an aqueous silver-salt bath containing one gram per liter of the silver salt.

(III) Ceramic nonmetallic workpieces are prepared for nickelor cadmium-coating in the following manner:

The baked-clay body is immersed in a solution of 10 grams nickel sulfate and/ or other nickel salts, e.g., nickel chloride, and 10 grams sodium hypophosphite in 100 cc. methanol and 10 cc. of organic acid, preferably formic acid, for a period of several seconds at a temperature between 40 and 90 C. to permit at least partial impregnation of the solution into the ceramic body. Upon removal from this solution, the body is heated to a temperature of about 200 C. until a dark or black coloration results. For the pretreatment of ceramic bodies in this fashion, approximately equivalent quantities of nickel salt and sodium hypophosphite may be used. Thereafter, the ceramic body may be chemically plated with bright nickel and cadmium coatings as set forth below. This pretreatment can be used for any refractory nonmetallic material which can be heated to 200 C. without damage and is suitable for porcelain and like materials having some degree of porosity.

CHEMICAL PLATING OF THE SUBSTRATE The plating baths prepared at (A) and (B) above are used to apply nickel and cadmium coatings to metal surfaces of metals which are more or less nobler than nickel and cadmium, respectively, which surfaces have been treated by conventional degreasing and deoxidation techniques. Suitable degreasing and deoxidation techniques involve abrading with abrasive papers or abrasive wheels, sandblasting, shotpeening and the like. Perfluoroethylene degreasing methods or hot-acid treatments may also be used. Thereafter, the metals may be etched with a mordant or an acid (e.g., concentrated sulfuric acid or chromic acid).

The mere immersion of a metal substrate into the nickel bath (A) or the cadmium bath (B) results in the deposition of approximately one micron of a smooth nickel or cadmium coating in a period of about 2 minutes. The plating continues to build up on an average of about 1 micron of nickel or cadmium for every 2 minutes of treatment, although the treatment time need not exceed 10 minutes for most effective results. In most cases, however, one micron of chemically deposited nickel suflices to provide a good metal surface upon nonconducting substrates.

During the chemical plating operation, there is a depletion of the nickel concentration and it is preferred to use automatic addition of nickel or cadmium salt and sodium hypophosphite to maintain the concentrations of these substances constant. However, since the bath provides a uniform deposition of nickel and cadmium over the period of about 10 minutes at which optimum deposition terminates, it is also possible to regenerate the bath upon the conclusion of the chemical precipitation operation. I also maintain the pH value between, say, 7 and 9 by the continuous addition of ammonium "hydroxide because the plating is accompanied by an increase in the acidity of the solution. I have found that the formation of ammonium compounds in the solution (e.g., ammonium sulfate in the case of a nickel sulfate bath) improves the buffer characteristics of the ammonium formate formed in situ. The concentration of this buffer serves to control the deposition rate of the nickel. The higher the buffer concentration, the slower is the precipitation of nickel.

Upon deposition of nickel or cadmium from the respective baths, the reduction of the nickel and cadmium ions to metallic nickel and cadmium respectively results in oxidation of the sodium-hypophosphite to the next oxidation state, i.e., the monosodium phosphite which eventually saturates the solution and precipitates. To effect removal of the precipitate, I circulate the bath continuously by pumping it through a filter and employ the pumping action and circulation to agitate the bath and maintain its homogeneity. Instead of the nickel sulfate, it is possible to use other nickel salts in the bath (A), including nickel chloride, although the chlorides are preferably avoided as has been indicated earlier.

The coating can be applied to nonmetallic substrates pretreated as indicated at (I), (II) and (III) above. The treatment for the thermoplastic fabrics and filaments can be used also for substances which are not refractory, e.g., ABS terpolymers (i.e., polymers of acrylonitrile, butadiene and styrene) in film or coating from or as solid bodies. Apart from ABS-type resins, polyester resins can be pretreated in this manner and coated with nickel or cadmium. It is found that a complete and uniform coating of ABS resins can be produced within a period of 20 to 45 seconds using the bath (A) above after pretreatment as indicated at (I) or (II). In fact, temperatures of 30 to 50 C. may be used to apply coatings in accordance with the present invention on nonmetallic substance while temperatures of 90 C. or more have been required in earlier chemical coating systems. Such high temperatures are disadvantageous when synthetic resins are involved.

For the coating of relatively refractory materials such as ceramic, stone, synthetic stone, Perlin and glass, the pretreatment (III) may be used. It has been found that the product resulting from this pretreatment followed by chemical plating in baths (A) or (B) yield a strongly adherent nickel or cadmium coating of good luster. After the coating step, the bodies are rinsed and dried.

I have found this latter technique is particularly desirable for the treatment of ceramic packing rings and like bodies for chemical reactors, heat exchangers and the like. It is especially valuable when the coated body is to be used as catalyst for chemical processes. Another advantage of the present method is that it provides a simplified handling of the plating bath at reduced cost since the chemicals used therein need not be of high grade but may be of ordinary technical purity. Moreover, the process can be controlled easily since only the concentrations need be considered and not the viscosity or any other factors and the concentration can be held Within narrow limits with conventional concentration-measuring and adhesive-feed devices. When metals more noble than nickel or cadmium are to be plated, it is advantageous at the commencement of the coating process to carry out precipitation of the more noble metal in contact with an iron, nickel or aluminum body as has been proposed elsewhere.

These modifications and others which will be readily apparent to those skilled in the art are intended to be included within the spirit and scope of the invention as claimed.

I claim:

1. A method of chemically plating a substrate with nickel or cadmium and which comprises the step of treat ing said substrate at a temperature between 40 and 70 C. With a chemical plating bath containing nickel sulfate or cadmium chloride and sodium hypophosphite in approximately chemically equivalent quantities, said bath being first brought to a pH of approximately 2 by the addition thereto of formic acid or an organic aliphatic acid of 3 to 5 carbon atoms and thereafter raised to a pH of about 6.5 to 9 by the addition of an alkaline substance selected from the group consisting of ammonium hydroxide and sodium hydroxide and stabilized with an ethylenediamintetraacetic acid compound.

2. The method defined in claim 1 wherein said temperature is about C.

3. The method defined in claim 1 wherein said organic acid is formic acid.

4. The method defined in claim 1 wherein the pH of said bath is maintained at about 8 to 9 during the treatment of said surface therewith, further comprising the step of circulating said bath and filtering precipitate therefrom during the circulation of the bath.

5. The method defined in claim 1 wherein said substrate has a metallic surface, further comprising the step of degreasing and deoxidizing said surface prior to its treatment with said bath.

6. The method defined in claim 1 wherein said substrate is a nonmetallic refractory, further comprising the steps of pretreating said substrate with a solution in an organic solvent of a nickel compound, hypophosphite ions and an organic acid, and heating the substrate upon the pretreatment thereof to a temperature of about 200 C.

7. The method defined in claim 6 wherein said organic solvent is methanol, the organic acid in said organic solvent is formic acid, and said hypophosphite is present in said organic solvent as a sodium hypophosphite, the pretreatment of said substrate being carried out at a temperature between 40 and C.

References Cited UNITED STATES PATENTS 2,532,283 12/1950 Brenner et al. 117-50 2,791,516 5/1957 Chambers et al. 117102(R) 3,148,072 9/1964 West et a1 1061 3,222,207 12/1965 Marshall 117-160 3,379,556 4/1968 Chiecchi 117-47(R) 3,403,035 9/1968 Schneble et al. 11747(R) OTHER REFERENCES Wein, S., Sensitizing Super-sensitizing and Iridizing Compounds, Part II in the Glass Industry, p. 708. December 1957.

ALFRED L. LEAVITT, Primary Examiner I. A. BELL, Assistant Examiner US. Cl. X.R. 

